I think their games do environmental storytelling better than most "open world" games - but obviously not as well as dedicated immersive sims, like Prey or Bioshock.
I fully agree that their main quests tend to be rather crap - or, at best, mildly interesting on occasion - but that's ok with me, as I spend very little time on those.
As for the locations, I think the reason they work for me is that you never actually know if it's going to be "one of the good ones" - or one of the more repetitive ones. There's enough quality in enough of their stuff that you never really know what to expect - and that's precisely what makes exploration worth it.
If you think about it, it's not that different from games like Baldur's Gate or Pillars of Eternity - when you go free roaming. The majority of the locations are essentially just mostly empty maps with a few potentially interesting encounters - but there's just enough there to make you go and check it out and sometimes you DO find a particularly interesting bit of quest or a nice dungeon.
For my part, I think Bethesda excel at doing distinct content within the formula of "huge open worlds" - and I can't think of other games of that kind of scope that even remotely provide as much interesting content - even if much of it IS repetitive and "samey".
As for the future, I'm still hoping they will tone down the amount of locations from hundreds to dozens - and, instead, focus all the content in fewer but more significant locations.
